1. Field of the Invention
This invention relates to a method for forming glassy coatings on metals. More particularly, the present invention is concerned with a method for forming a hard silicate coating on metals. Still more particularly, this invention is concerned with an electrolytic method for forming an impervious corrosion-resistant silicate coating on metals, especially aluminum.
2. Description of the Prior Art
Electrolytic procedures for depositing silicate coatings no metals are known. For example, Czechoslovakian Pat. No. 104,927 issued Sept. 15, 1962, to Hradcovsky and Belohradsky discloses an electrolytic method for depositing a silicate coating on aluminum employing as the electrolyte a weekly alkaline aqueous bath composed of 10 to 15% sodium or potassium silicate and a hardener such as 3% ammonium molybdate. When the anode is aluminum and the cathode is iron or nickel, and an increasing voltage is impressed across the electrodes, a silicate coating begins to form at a voltage of about 300 volts. The coating formed by the method described in said Czechoslovakian patent was intended to have a weak break-down voltage, and is a highly porous coating.
More recently, Hradcovsky and Kozak in U.S. Pat. No. 3,834,999, issued Sept. 10, 1974, disclosed forming a non-porous glassy protective coating on various rectifier metals, including aluminum, by an electrolytic process employing as the electrolyte a bath containing an alkali metal silicate and an alkali metal hydroxide. The metal is made progressively more anodic until at 250 volts a discharge occurs and deposition of the coating begins. Voltages of at least about 400 volts are required to obtain a satisfactory coating and coatings of about 1mm thickness may be readily prepared by this method. The silicate electrolytes disclosed are relatively dilute, and are rendered alkaline with an alkali metal hydroxide, with an alkali concentration of about 15% being desired to achieve the hardest coating. The intent of this process was to provide a glassy, adherent, corrosion-resistant protective layer on the metal, in contrast to the porous layer of the Czechoslovakian patent.
Still later, it was discovered that rectifier metals may be provided with durable silicate protective coatings by electrolysis in an electrolyte comprising a strongly alkaline bath containing an alkali metal silicate, an alkali metal hydroxide and an oxyacid catalyst. In this process, a voltage of at least 220 volts is required to deposit a coating. When the voltage exceeds about 220 volts, sparking occurs, causing deposition of the desired coating. However, depending upon the bath composition, voltages of up to 1,500 volts may be required to deposit a satisfactory coating. As in the prior process, the silicate content of the electrolyte bath is relatively low. In particular, the silicate concentration is 2.5 to 200 grams per liter, and preferably 10 to 50 grams per liter, especially 15 to 25 grams per liter. See U.S. Pat. No. 3,832,293.
Although the methods of the above-described United States patents impart glassy, protective silicate coatings which are superior to the porous coatings of the Czechoslovakian patent, they require substantial voltages and involve a relatively high consumption of electricity. Furthermore, substantial periods of time on the order of several minutes or more are required to build up coatings of the desired thickness following the teachings of these patents. In addition, certain of the electrolyte baths disclosed by these references, particularly baths containing sodium silicate, are unstable with resulting precipitation and change in electrolyte content so that the bath may not be reused over any substantial period of time.